Astronomical seeing

Astronomical seeing is the blurring and rapid distortion of images of celestial objects caused by turbulence in Earth’s atmosphere. It limits the finest detail ground-based telescopes can resolve.

Atmospheric turbulence: Irregular, changing motion in the atmosphere that mixes air with different temperatures and velocities. Turbulent air creates the shifting refractive-index variations that distort incoming starlight.
Seeing monitor: An instrument or method that estimates atmospheric image blur at an observing site. Continuous measurements reveal whether conditions support fine-resolution observations.
Diffraction limit: The finest angular detail an optical system can resolve because of light's wave nature and its aperture. Seeing often dominates ground-based images, but diffraction sets the limit when atmospheric blur is corrected.
Angular resolution: The smallest angular separation at which an imaging system can distinguish two sources. Atmospheric blur sets a practical resolution floor for many ground-based observations.
Optical wavefront: A surface of equal phase across a propagating light wave. Seeing arises when turbulence deforms the wavefront arriving at a telescope.
Differential image motion monitor: An instrument that estimates astronomical seeing from differential motion between stellar images. It provides a standard way to measure seeing without relying on a full telescope image.
Space telescope: A telescope operating beyond Earth's atmosphere to observe astronomical objects. Above the atmosphere, space telescopes avoid atmospheric seeing altogether.
Astronomical site selection: The choice of locations for observatories based on atmospheric, geographic, and logistical conditions. Sites with weaker turbulence can deliver sharper images and reduce correction demands.
Fried parameter: A measure of the atmospheric coherence length over which wavefront distortion remains relatively small. It links turbulence strength to the angular blur expected at a given wavelength.
Lucky imaging: A method that selects and combines the sharpest short-exposure astronomical images. It exploits brief moments when atmospheric distortion is unusually weak.

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